1. Field of the Invention
The present invention relates to a method for exposure dose setting in photolithographic instruments.
2. Description of the Prior Art
Photolithographic instruments are used in the manufacture of integrated circuits. In this technology, wafers of silicon are coated with a photoresist material. When exposed to ultraviolet light the photoresist material is polymerized into a hard tenacious coating and the unexposed portions of the layer may easily be removed by means of a solvent or developer. By means of a mask or master which is opaque except for a pattern of transparent lines, the photoresist coating may be exposed to light in a pattern conforming to a circuit element or elements. A wafer may be exposed to successive masks, each of which exposes a new photoresist coating in a different pattern on its surface, forming additional circuit elements. The optical images formed on the wafer in successive exposures must, however, be closely controlled in relative position, for example, by use of patterning exposure tools, such as projection printers, 1x scanning tools, reduction scanning tools, stepping and contact instruments, broadly defined as photolithography instruments.
Typical photolithography projection printers used in exposing photoresist in such a manner are described in U.S. Pat. Nos. 4,011,011, to Hemstreet et al., 4,068,947 to Buckley et al., 3,951,546 to Markle, 3,937,556 to Newell, 3,952,217 to Rawllings, 4,248,508 to Watkin, 3,957,364 to Markle et al., and 4,240,702 to Casas. These printers operate without projection lenses made up of refractive elements, instead exposing the wafer through a mask by scanning the mask with an illumination source, preferably, a UV (Ultra Violet) source. Control of exposure times in patterning exposure tools is obtained by control of the speed of the scanning motion and of the width of the area illuminated on the mask, or by varying shutter speeds.
It is necessary to control the exposure (the amount of energy reaching the photoresist) carefully in order to ensure the quality of the developed image. As the amount of exposure of a photoresist layer to UV light is increased, the photoresist layer is progressively bleached. It is well known in the art that good process lithography occurs at definite levels of bleaching, for example, at about 37% relative photoactive compound (PAC) bleaching for a 0.27 molal diazoketone/novolac positive resist. However, because of fluctuations of scan speed and lamp intensity, it is quite difficult to reproduce the optimum dose on a given day using known photolithographic instruments which have no internal dose monitor.
Previous methods of dosimetry for UV photolithographic instruments are well known. For instance photodiode probes responding to all emission lines for the mode (i.e., UV-4, UV-3, or UV-2) being employed may be used. It is also known to use commercial dosimeter film, whereby the film, in the form of a strip taped onto the wafer, is exposed to varying dose amounts. However, because of the limited light sensitivity of available films, this method cannot be used in all exposure modes, that is, for all wavelengths of ultraviolet light. Furthermore, a calibration curve must be generated for each lot of film since lot-to-lot variations may occur. Moreover, this process, which involves darkening of a light-sensitive material rather than PAC bleaching, is only indirect.
Another known method of dosimetry involves exposing a test photoresist layer through a mask which provides images of varying percent transmittance. After development, the photolithographic instrument dose is approximated for a known process window via SEM (Scanning Electron Micrograph) image analysis. This method, however, is not quantitative and is dependent on the type of photoresist, developer and processing. It also requires a scanning electron microscope.
The known end point detection method of dosimetry involves the exposure of a test photoresist layer through a blank quartz mask after which a laser endpoint detection is run to determine dissolution curves. The photolithographic tool dose is approximated for a known process window via the laser endpoint detection dissolution data base. This method has the disadvantages that a laser endpoint detection system is required, it is not quantitative and is dependent on the type of photoresist, developer and laser endpoint detection procedure.
Photoresist image analysis is also known as a method of dosimetry. After exposure and development, the relative dose is estimated as a function of line width from optical or electron microscopy techniques. A photolithographic tool dose can be approximated from an SEM image data base. Disadvantages of this method are that it requires a SEM or other microscope is not quantitative, and is dependent on the type of photoresist, developer and processing.
Accordingly, an object of the invention is to provide a method for determining the mid-, near- or deep UV exposure dose of a patterning exposure instrument as a function of scan speed or shutter speed.
Another object of the invention is to provide a method of dosimetry which has low measurement error and which offers a measurement directly correlated with photoresist lithography usage.
Yet another object of the invention is to provide a method for matching a photolithographic instrument performance with each lot or loading of photoresist.
A further object of the present invention is to provide a simplified method of dosimetry using photoresist coated discs which do not require the use of adhesion promoters or prebake.